Communication system



W. E. HARDING COMMUNICATION SYSTEI Jan. 23, 1940.

Filed April 15, 1938 2 Sheets-Sheet 1 INVENTOR.

ATTORNEYS.

Jan. 23, 1940.

w. E. 'l -IARDING COIMUNICATION SYSTEI Filed April 15, 1938 2Sheets-Sheet 2 INVENTQR. W120? fflwpom a:

ATTORNEYS.

Patented Jan. 23, 1940 UNITED STATES PATENT OFFICE 17 Claims.

My invention relates to communication systems, such as telephonicrailway-dispatching systems, in which a plurality of transmitters arelocated at intervals along a telephone line, and to an automaticvolume-control apparatus for use in such systems as well as in othersituations.

. More specifically, my invention relates to a telephonicsystem-incorporating one or more line boosters. In such a system, thebooster receives current from the line, amplifies'it, and returns theamplified currentto the line. If the booster is located at one of theregular stations along the line, it may have associated with it a microphone, or other transmitter, and a sound-reproducing device, such as aloud speaker or telephone receiver.

It is the object of my invention to produce for 'use in such systems abooster having advantages over prior boosters of which I am aware. Morebooster in which any possibility of feed-back of amplified current intothe amplifier is minimized or practically eliminated and in whichvariations the booster in such a way that it will not feed into theautomatic volume-control apparatus any of the sound it receives.

In carrying out my invention, I provide in the booster a pair ofbalanced circuits having a common or bridge portion, the'latter beingcoupled to the input of an amplifier of any desired gain. The line iscoupled to the balanced circults in such a way that current from theline is applied to but one of the balanced circuits, and therefore flowsthrough the bridge. Output from the amplifier is divided-equally betweenthe two balanced circuits and is therefore without any effect on thecommon or bridge portion thereof; but the coupling between the line andthe two balanced circuits is such thatamplifled current is returned tothe line. Coupled to the two balanced circuits I provide a compensatingcircuit that acts to reduce the extentto which a: changes inline-impedance affect the balance of specifically, it is my object toproduce a linein line-impedance will not adversely affect oper-Application April 15, 1938, Serial No. 202,196

the balanced circuits with respect to amplified current.

A portion of the output of the amplifier associated with the booster maybe supplied to a loud-speaker or other sound-reproducing device, 5desirably through an automatic volume-control apparatus. Whenconstructed in accordance with my invention, such an automaticvolumecontrol apparatus includes. a pair of amplifiers whose outputs areconnected, in opposed pushpull relation, to the loud-speaker. The inputcurrent of one of such amplifiers isv made to vary proportionately withthe input of the automatic volume-control apparatus, while the input tothe other amplifier is made torvary in accordance with the amount bywhich the input to the automatic volume-control apparatus exceeds somepredetermined value. This result is accomplished by an arrangement ofbalanced circuits to be described in detail hereinafter. 20

If a microphone or other transmitter is to be associated with thebooster, its output is divided equally between the two balanced circuitsof the booster so that it is not responsible for any current in thebooster-bridge. In this way current from the microphone is kept out ofthe amplifierinput.

The accompanying drawings illustrate my invention: Fig. 1 is adiagrammatic view of a complete station showing the booster circuits;Fig. 2 is a diagrammatic view of the preferred form of automaticvolume-control apparatus; and Figs. 3 and 4 are fragmental views ofautomatic volume-control apparatus illustrating two differentmodifications thereof.

A booster constructed in accordance with my invention embodies a pair ofassociated balanced circuits l and I0 having a common portion, or bridgell. Connected in the bridge II is the primary I! of a transformer whosesecondary I3 is connected to the input of an amplifier ll of any desiredconstruction. The output circuit of the amplifier 14 includes theprimary l5 of a transformer having a secondary IS. A portion of thetransformeresecondary is included in a circuit II which also includesthe primaries l8 and I8 of two transformers whose secondaries l9 andi9". are connected similarly in the balanced circuits l0 and I0respectively. The windings l8, l8, l9. and I9 may form the four equalwindings of a common type of transformer employed in telephone circuits.The circuit ll mayinclude a switch for the purpose .of cutting out thebooster when desired.

In addition to windings l9 and IS, the balanced circuits l8 and III alsoinclude respectively 10 pling transformer having one winding 3i,connected across the line 38, and-a winding 32 whose terminals areconnected to opposite ends of the windings 21 and 21' respectively. Theother two ends of the windings 21 and 21 are connected 5 togetherthrough one winding of a transformer 33 whose other winding is connectedin series with a variable impedance 34. Thus a closed circuit isprovided including, in the order named, the winding 21, the winding 32of the coupling pedance 34. On the other hand, if compensation werecomplete, line current in windings 28' and 21' would induce in circuit31 a current substantially balancing the circuits l8 and 18 to linecurrent, thus keeping line current out of the bridge I I and thereforeout of the amplifier input. The presence of the impedance 38, however,lowers the degree of compensation effected by the circuit 31. Theimpedance 38 is ordinarily adiusted to the lowest value which will notobjectionably interfere with the transfer of energy from the line to theinput of the amplifier i4; and I find in practice that with theimpedance 38 so adjusted no noticeable feedback of amplified currentinto the amplifier input will occur even under wide variations in lineimpedance.

The impedance 38 desirably includes a phasecorrecting condenser tocompensate for the inductance of the windings 28 and 28'.

m transformer, the winding 21', and the transformer 33. The two windings28 and 28' are interconnected in opposed relation in series to form abridge extending across the circuit just described from a point betweenthe windings 21 a8 and 32 to a point between the winding 21 and thetransformer 33.

The two windings 28 and 28' are connected in opposed relation in seriesin a compensating circuit 31 which includes a variable impedance 38.

go In the booster described, line current, applied through thetransformer 3l -32, is divided into two components which flow inopposite directions through the windings 28 and 21 and in the samedirection through the windings 28' and 21'. Because of its divisionbetween and opposite fiow through the windings 28 and 21, this currentis noninductive with respect to the circuit II), but does create aproportional induced current-in the circuit 18' (which includes thebridge ll). Through the coupling transformer i2-i3, current in thebridge II is supplied to the amplifier The amplified current in thecircuit l1, which is coupled to the amplifier-output by the transformerI8-I8, flows through the transformer.

windings l8 and I8 and generates equal voltages across the windings i8and 18. These voltages are equal; and if the line impedance reflected 5through transformer 3l-32 is equal to the irripedance 34 as reflectedthrough the transformer 33, the voltage across the winding 21 will equalthe voltage across the winding 21', and no current will flow through thebridge consisting of the windings 28 and 28. As the result of this, thevoltage drop across the windings 28 and 28' will be equal, as will alsothe voltages across the windings 28 and 28'. As the windings 28 and 28'are connected in oppom relation, no current will fiow in thecompensating circuit 81.

The compensating circuit 31 tends to equalize the respective eflects oncircuits l8 and III of currents in the windings 28, 28', 21, and 21.Therefore, if as a.result of a lack of balance between line impedanceand the impedance 34 the current flowing in winding 21 is unequal-to thecurrent flowing in winding 21', there will be induced in the circuit 31a current which will tend to compensate for the effect of inequalitiesin the current in therespective windings 21 and 21'.

If there were no impedance in the compensating circuit, compensationwould be substantially complete (except for transformer losses) and thecircuits l8 and 18' would always be substantially balanced to amplifiedcurrent irrespective of discrepancies between line impedance and the im-80 It may be desirable to provide in the bridge including the windings28 and 28' a condenser 43 of a capacity such as to present relativelyhigh -impedance to low-frequency signal currents.

This prevents shunting of low-frequency currents, such as bell-ringingcurrents, and also suppresses low-frequency oscillation.

If reproduction from the line is desired at the station a portion of theamplified current from the transformer-secondary I8 is supplied to asound-reproducing device such as a loud-speaker 48, desirably through anautomatic volume-control apparatus 48. The apparatus which I prefer touse is illustrated in detail in Fig. 2 and comprises two three-elementvacuum tubes 88 and 88' whose plate circuits are connected in opposedpush-pull arrangement and coupled by a transformer 8| to the speaker 48.The grid circuit of the tube 88' is coupled, as through a couplingtransformer 83, with an intermediate circuit 84 includingthe'secondaries 88 and 88 of two transformers A and B. The primaries ofthe two transformers A'and B are connected respectively in two balancedcircuits 81 and 88 which have in common a bridge 88. The balancedcircuits 81 and 88 include complementary portiohs of an impedance 88 toan intermediate point of which one end of the bridge 88 is connected. Inaddition, the balanced circuits 81 and 88 include respectively thetransformer secondaries 8| and 82 whose primaries are connected inseries to the input circuit 83, which in this instance includes thetransformer-secondary l8;

The coupling between the intermediate circuit 84 and the balancedcircuits Hand 88 is such that changes in the current flowing in theintermediate circuit 84 will aflect diflerently the efl'ectiveimpedances in the respective circuits 81 and- 88. To secure this result,I prefer to make the transformers A and B of different ratios and toconnect their secondaries 88 and 88 into the circuit 84 in such a waythat their secondary voltages are opposed to each other. As indicated inthe drawings, the transformer A is a step-up transformer while thetransformer B has a oneto-one ratio, and the two secondaries 55 and 55are connected together so that the .efiective E. M. F. causingcurrent-flow in the circuit 54 is the difierenc between the respectivevoltages across such transformer-secondaries.

The grid circuit of the vacuum tube 50 is coupled by a transformer 64 tothe secondary of a second transformer 55 whose primary is connected inthe bridge 59. As a result; since the two amplifiers are connected inopposed relation, their joint output will depend upon the difference invalue between the currents flowing in the circult 54 and in the bridge59 respectively. Upon any change of the current flowing in the inputcircuit 63, the voltages across the transformer secondaries 6i and 62(which voltages are responsible for current flow in the balancedcircuits 51 and 58) will be changed in the same sense, as will also thecurrent flowing in the intermediate circuit 54. If it be assumed thatthe change in current in the input circuit is an increase, an increasewill also occur in the current flowing in the intermediate circuit 54,which includes the secondaries 55 and 56 of the transformers A and B.The increase in current flowing through the transformer-secondary 55will produce an effective decrease in the impedance of the primary ofthe transformer A. However, since the intermediatecircuit current flowsthrough the transformersecondary 58 in a direction opposite to that inwhich it flows through the secondary 55 of the other transformer, theincrease in such. intermediate-circuit current increases the effectiveimpedance of the primary of the transformer B. The decrease in effectiveimpedance in the primary of the transformer A and the increase ineffective impedance of the primary of the transformer Bwhich thusaccompany an increase in the current flowing in the circuit 54 result inan increased current-flow through the bridge 59. As

. the bridge 59 is coupled to the grid circuit of the vacuum tube 50,that tube, acting through its plate circuit and the transformer5l,compensates for the increase in current flowing in the plate circuit ofthe tube 59' which results from the increase in current flowing in theintermediate circuit 54.

Similarly, a decrease in the current flowing in the input circuit willbe accompanied by decreases in the strengths of the respective currentsin the intermediate circuit 54 and in the bridge 59.

In the device as so far described, the strength of current flowing inthe intermediate circuit 54 may be expressed as k1, where -I is thestrength of current in the input circuit 63 and k is a constantdepending upon the characteristics of the electrical apparatus; and thestrength of the current in the bridge 59 may be expressed as k'(IIo),where I has the same value as before, It is another constant dependingupon.

electrical characteristics of the apparatus, and I0 is the value of thecurrent in the input circult 53 when no current is flowing through thebridge 59.

If m represents the gain provided by the amplifler including the tube 50and m represents the gain provided by the amplifier including the tube50', the net output of the two amplifiers will be represented by theequation:

(1) Output=mkIm'Ic'(I--lo) By suitable adjustment of the amplifiers, theconstant m may be made, equal to Substituting this value for m inEquation 1. that equation becomes:

(3) Output=mlcIo As the quantities m, k, and I0 are all independent ofI, it is apparent that fluctuations in strength of the input current Iwill not aifect the net outa put of the two amplifiers if the respectivegains which those two amplifiers provide is properly selected withregard for other electrical characteristics of the apparatus.

To effect satisfactory regulation of the constants k, k, and I0 in theabove equations two circuits in addition to those already described maybe employed. One of these circuits, indicated at in Fig. 2, includes avariable impedance II and similar windings l2 and 13, which windings areembodied respectively in the transformers A and B, and acts as a loadcontrol circuit. The other, indicated at 14, includes a variableimpedance and a winding 16, the latter embodied in the transformer B andoperating, as regulated by adjustmentof the impedance 15, to determine,for any adjustment of the impedance ll the value of the current 10 incircuit 63 at which no current flows through the bridge .the drawings astwo two-winding transformers 5| and 62.

With the arrangement illustrated in Fig. 2, the connection between thebridge 59 and the resistance 60 will be adjusted to create a conditionof balance between the circuits 5! and 58 when circuits 54 and H areopened. The impedances II and I5 and the amplifiers are then adjusted togive the desired results. The preferred method of adjustment comprisesmaintaining in the circuit 63 a current of mean value while adjustingthe impedances H and I5 and the amplifiers to such a condition that eachamplifler will be operating near the middle of the straight portion ofits response curve when desired output to the speaker is obtained.

The modified arrangement illustrated in part in Fig. 3 is identical withthat shown in Fig. 2 except that the transformer-windings and 55 areconnected in opposed relation in the circuit 10 instead of in thecircuit 54 while the circuit 54 includes the transformer-windings I2 and13. This arrangement employs the same basic principle as does that ofFig. 2, the only difference between the two arrangements being that inthat of Fig. 3'it is current in the circuit 10 rather than current inthe circuit 54 which is employed to control the state of balance orunbalance of the two circuits 51 and 58 and hence the strength ofcurrent in the bridge 59.

In the arrangement-illustrated in Fig. 4, the transformer-winding 15 andthe circuit'14 associated with it are eliminated and the transformerwindings 55 and 55 are connected in parallel,

rather than in series, in the circuit 54. An adiustable impedance .0 maybe connected in series with the winding 56 to perform the same functionas is performed by the impedance It of Fig. 2. In such an arrangement,the current flowing through winding so will not be the same as thatflowing in winding 58, but it will vary in the'same sense as the currentin winding it varies and thus will co-operate with the current inwinding SI to control-the degree of unbalance of the circuits l1 and 88.

If, for any reason,- there should be any differenoe inphase-relationship between currents in the secondaries of thetransformers 53 and 84 it may be corrected for by appropriate use of oneor more condensers (not shown) in the circuit l4 and bridge 59.

Many modifications and arrangements of my invention other than thosespecifically illustrated -maintain a substantially constant sound-outputirrespective of the loudness of the speaker's voice.

I claim as my invention:

1. A booster for telephone lines or the like; comprising a pair ofbalanced circuits including a bridge portion common to both; anamplifier having an input circuit coupled to said bridge portion and anoutput circuit coupled independently to said two balanced circuits; twofourwindlng transformers having their primaries connected respectivelyin said balanced circuits;

an intermediate line-circuit including, in order, a secondary winding ofone of said transformers, an impedance, and a secondary winding of theother transformer; the line being coupled to said intermediate circuitat a point therein between the said two transformer windings; aconnection including another secondary winding of each .transformer andextending across said intermediate circuit from a point between one ofthe windings therein and said impedance to a point between the otherwinding therein and the coupling with the line; and a compensatingcircuit including an impedance and the remaining windings of saidtransformers connected in series and in opposed relation.

2. A booster as set forth in claim 1 with the addition that theimpedance in said intermediate circuit is approximately equal to meanline impedance.

3. A booster for telephone lines or the like; comprising a pair ofbalanced circuits including a bridge portion common to both; anamplifier having an input circuit coupled to said bridge portion and anoutput circuit coupled independently to said two balanced circuits; aninterme-' diate circuit coupled to the line and to each of said balancedcircuits to create in said bridge portion a current proportional toincoming line currents; and a compensating circuit including windingsconnected oppositely in series; said windings being coupled respectivelyto said balanced circuits and also to said intermediate linecircuit.

4. A booster for telephone lines-or the like;

- aromas comprising a pair of balanced circuits including a bridgeportion common to both; an amplifier having an input circuit coupled tosaid grldge portion and an output circuit coupled independently of saidtwo balanced circuits; a pair of windings coupled to each of saidbalanced circuits; an intermediate circuit including, in order, onewinding of one pair of windings, an impedance, and one winding of theother pair of windings; the line being coupled to said inter-' 5. Theinvention set forth in claim 4 with the addition that said impedance isapproximately equal to line impedance.

6. A booster for telephone lines or the like; comprising a pair ofbalanced circuits including a bridge portion common to both; anamplifier having an input circuit coupled to said bridge portion and anoutput circuit coupled independently to said two balanced circuits; anintermediate line-circuit divided into two arms by a cross-connection;one arin of said intermediate circuit being-coupled to the line and theother arm containing an impedance approximately equal to line impedance;each arm of said intermediate line-circuit being coupled to saidbalanced circuits by couplings of different effectiveness; saidcross-connection being oppositely coupled to said balanced circuits.

7. A booster for telephone lines or the like; comprising a pair of",balanced circuits including a bridge portion common to both; anamplifier having an input circuit. coupled to said bridge portion and anoutput circuit coupled independently to said two balanced circuits; anintermediate line-circuit divided into two arms by a cross-connection;one arm of said intermediate circuit being coupled to the line and theother arm containing an impedance approximately equal to line impedance;eacharm of said intermediate line-circuit being coupled to said bal-'anced circuits by couplings of different effectiveness.

8. The invention 'set forth in claim 7 with the addition of a microphonesimilarly coupled to said two balanced circuits.

' 9. In aline bocster,apairofbalancedcircuits having a common bridgeportion, an amplifier having its input coupled to said bridge portionand its output divided equally between said balanced circuits, a dividedline circuit having two arms coupled respectively to said balancedcircuits, and a compensating circuit coupled to eacharmofsaidlinecirciutandtoeachofsaid balanced circuits and including animpedance.

10.Inalinebooster,apairofbalanced circuits having a common bridgeportion, an amplifier having its input coupled to said bridgeportion andits output divided equally between said balancedcircuits, a divided linecircuit having two arms coupled respectively to said balanced circuits,and a circuit coupled to eacharmofsaidlinecircuitandtoeachofsaidbalanced circuits. 4 Y

11. In a. line boostenapair of balancedcircuits 7 having a common bridgeportion, an amplifier having its input coupled to said bridge portionand its output divided equally between said balas between said balancedcircuits the eifeot of un-- equal impedances in the arms of said linecircuit.

12. In a line booster, a pair of balanced circuits having a commonbridge portion, an amplifier having its input coupled to said bridgeportion and its output divided equally between said balanced circuits, adivided line circuit having two arms, one of said line-circuit armsbeing coupled directly to one of said balanced circuits and indirectlyto the second balanced circuit, the other of said line-circuit armsbeing coupled directly to the second balanced circuit and indirectly tothe first balanced circuit.

13. In combination, a speaker, a pair of vacuum tubes havinggrid-circuits and plate circuits with the latter coupled in opposedpush-pull arrangement to said speaker, an intermediate circuit coupledto the grid-circuit of one of said tubes, a pair of balanced circuitshaving a common bridge portion coupled to the grid-circuit of the otherof said tubes, apair of transformers of different ratios, each of saidbalanced circuits including the primary of one of said transformers, thesecondaries of said transformers being connected in the latter coupledin opposed push-pull arrangement to said speaker, an intermediatecircuit coupled to the grid-circuit of one of said tubes, 2.

pair or balanced circuits having acommon bridgeportion coupled to thegrid-circuit of the other of said tubes, a pair of transformers havingtheir secondaries connected respectively in said balanced circuits, aninput circuit including the primaries of said transformers, anddissimilar coupling means coupling said intermediate circuit with saidbalanced circuits respectively. I

15. In combination, a speaker, a pair of vacuum tubes havinggrid-circuits and plate circuits with the latter coupled in opposedpush-pull arrangement to said speaker, an intermediate circuit coupledto the grid-circuit of one of said tubes,

proportional to the current in said input circuit.

16. In combination, aspeaker, a pair of vacuum tubes havinggrid-circuits and plate circuits with the latter coupled in opposedpush-pull arrangement to said speaker, an input circuit, means'forvarying the current in one of said grid circuits in proportion tovariations in the current in said input circuit, and means for varyingthe current in the other grid circuit in proportion to the extent bywhich the current in the input circuit exceeds a predetermined value.

17. In combination, an input circuit, an output circuit, a pair ofamplifiers connected in opposed push-pull relation and feeding saidoutput circuit, means for supplying one of said amplifiers with currentproportional to current in said input circuit, and means for supplyingthe other amplie her with current proportional to the amount by whichcurrent in said input circuit exceeds a predetermined value.

WILLIAM E. HARDING.

